1. Field of the Invention
The present invention relates to the preparation of silyl-containing materials having alkoxy end groups. More specifically, this invention relates to the use of organolithium reagents in the preparation of organopolysiloxane materials having alkoxy terminal groups. These materials can also have olefin functionality, thereby allowing for dual cure by means of moisture and photo curable mechanisms.
2. Description of the Prior Art
It is known that alkoxy terminated polymers can be prepared by reacting di-, tri or tetralkoxysilanes with diorganosiloxanes having silanol terminal groups at each end of the polymer chain. This reaction requires the use of specific catalysts such as amines, inorganic oxides, potassium acetate, organic titanium derivatives, titanium/amine combinations, carboxylic acid/amine combinations as well as carbamates and oxime-containing organic compounds. All of these catalyst systems have significant drawbacks, however. For example, amine catalyst systems are particularly slow and this lack of speed is often further exaggerated, depending upon the reactivity of the particular alkoxysilane. In addition, amines and carboxylic acid catalysts are corrosive and require special handling and removal processes once the reaction has proceeded to the desired state of completion. Catalyst reaction residue is particularly detrimental on the storage stability of the organosiloxane fluids and the properties of the resulting crosslinkable product. Removal of these catalysts is often difficult, requiring extra steps which are laborious and costly. Furthermore, many of these catalysts are particularly offensive to the body, giving off unpleasant odors and being dangerous to eyes and skin.
Organic titanium catalysts such as titanium tetra isoproprionate have also been commonly used for these type of reactions but suffer from the deleterious effect of "thick-phasing", a result of titanium-silicon complexing. Before the intended reaction between the silanol-terminated organopolysiloxane and the alkoxy silane is complete, a thick gel-like phase forms, requiring additional shear forces and energy from the mixing blades to overcome this phase and allow the reaction to proceed. Thick-phasing is a particularly difficult problem when industrial-sized batches of the polymer are being prepared and the shear forces necessary to overcome thick-phasing are high.
More recently, U.S. Pat. No. 5,079,324 to Cocco et al. discloses the use of lithium hydroxide as a catalyst for the reaction of diorganosiloxanes having terminal silanol groups, with a polyalkoxysilane of the formula: EQU (R").sub.c (R').sub.b Si(OR"').sub.4-(a+c)
wherein R' and R" may be, for example, a monovalent hydrocarbon radical that may contain functional groups C.sub.1-13 ; and R"' is an aliphatic radical C.sub.1-8.
However, lithium hydroxide is an inorganic solid which requires use of a polar solvent such as methanol to introduce it as a solution into the reaction. Due to the presence of methanol, this catalyst system has the distinct disadvantage of being continually regenerated in the form of lithium methoxide. The resultant polymer product exhibits a rapid lowering of viscosity due to attack of the regenerated lithium catalyst upon the siloxane. The viscosity drop, although generally rather immediate, becomes more pronounced over time and shelf life is therefore greatly affected. Furthermore, subsequent curing of this functionalized polymer is also deleteriously affected. It is believed that the viscosity drop is directly related to cleavage of the siloxane bond by the regenerated lithium, resulting in a significantly lower molecular weight and product instability.
The use of butyl lithium in combination with hexamethylphosphoroamide as initiators for the oligomerization of hexamethylcyclotrisiloxane with methylmethoxysilanes is disclosed in Eur. Plym J., Vol. 21, No. 2, pp. 135-140, 1985 entitled "The Anionic Oligomerization of Hexamethylcyclotrisiloxane with Methylmethoxysilanes". This reference discloses the use of butyl lithium as well as inorganic bases as a means to ring-open hexamethylcyclotrisiloxane in the presence of methylmethoxysilane.
EPA 362710 to Toray Silicone Company discloses a method of synthesizing a diorganopolysiloxane in which a polymerizable functional group is present at one or both molecule chain ends. The synthesis involves the opening of a cyclotrisiloxane using an alkali metal silanolate, which bears the polymerizable functional group, as a polymerization initiator in the presence of an organosilane or organopolysiloxane. This reference states that if the reaction is run at equilibrium, a mixture of products is produced, i.e. polymer with functional groups at both ends and polymer without functional groups at either end. This reference does not teach a method of endcapping with alkoxy groups on both ends of the polymer but rather a polymerization reaction using cyclic trisiloxane as the living polymerization monomer.
U.S. Pat. No. 3,878,263 to Martin discloses a polymerization reaction which uses organo-lithium catalysts in aprotic solvent as well as inorganic lithium catalysts to polymerize a cyclic organopolysiloxane with an acrylate functional silane or siloxane. This reference begins with different starting materials from the present invention to perform a ring-opening reaction, i.e. siloxane cleavage. The resultant polymers have only one end terminating in an alkoxy group, the other end terminated with the acrylate group. The polymerization reaction is controlled by the terminating methacryloxy group. Those catalysts disclosed include lithium alkoxide compounds such as lithium methoxide; lithium alkyls such as ethyl lithium, isopropyl lithium, n-butyl lithium, vinyl lithium; lithium aryls such as phenyl lithium; lithium hydride, lithium aluminum hydride, lithium silanolate and lithium hydroxide.
There is a definite need for a process for making viscosity stable alkoxy silyl-containing materials, such as alkoxy terminated organopolysiloxanes, which is noncorrosive, fast and does not suffer from the unpleasant odors and other disadvantages of the prior art catalyst systems. Additionally, there is a need for such compositions which exhibit stable shelf life and can be useful for making such products as sealants, adhesives, coatings and the like.